Photolithography is a lithographic technique using an image produced by photography for printing on a print-nonprint surface. With this technique pulsed light sources can be used to expose a light sensitive surface. Typically, pulsed light sources have a pulse-to-pulse energy fluctuation. This pulse-to-pulse fluctuation makes accurate exposure difficult. Pulsed sources are additionally troublesome because the mean energy per shot may drift with time and other factors. This compounds the difficulty in maintaining the correct exposure. Additionally, the correct exposure may not require an integal number of shots which contributes to the already inaccurate exposure. Therefore, there are problems in maintaining correct exposure when using pulsed light sources.
A common technique used to control the exposure dose is to increase the number of shots or light pulses needed to expose the light sensitive surface. With a large number of shots the fluctuation in each shot is not as critical in maintaining the proper exposure. But, increasing the number of shots increases the time needed for exposure as well as in most situations requires a large attenuation of the pulsed light source. Increasing the number of shots required for exposure can control exposure to within acceptable limits, but is very inefficient.
Attempts have also been made to control the average pulse energy of the pulsed light source so that it is an exact sub-multiple of the required dose for exposure. This technique requires a pulsed energy source with little noise or pulse-to-pulse energy fluctuation. Even with a low noise pulsed light source the number of shots required to obtain a reasonably accurate exposure dose is large.
While the above techniques have greatly increased the use of pulsed sources in photolithography and have achieved some degree of control over the exposure dose they have proven to be inefficient exposure systems. The exposure necessary to obtain a reasonably precise exposure dose is time consuming and shortens the lifetime of the pulsed source which is usually measured by the number of shots fired.